3,126 research outputs found
IODP Exp. 359 – Maldives Monsoon and sea level - Luca Lanci
No full textExp. 359 – Maldives Monsoon and sea level - Luca Lanci</p
Data for: Magnetic properties of Early Pliocene sediments from IODP Site U1467 (Maldives platform) reveal changes in the monsoon system
No full textPaleomagnetic and rock-magnetic data from IODP Site U1467 from "Magnetic properties of Early Pliocene sediments from IODP Site U1467 (Maldives platform) reveal changes in the monsoon system” by L. Lanci, E. Zanella and Exp. 359 member
Ice magnetization in the EPICA-Dome C ice core: preliminary results
No full textWe measured the Isothermal Remanent Magnetization (IRM) and coercivity of remanence (Hcr) of fine magnetic particles included as dust in ice samples from the EPICA ice core (East Antarctica). Atmospheric aerosol contains a significant fraction of highly magnetic iron oxides whose magnetization can be measured directly on ice samples (Lanci et al., 2001). The concentration and mineralogy of the magnetic fraction in polar ice can be estimated through standard magnetic methods used in environmental magnetism; this information on a novel property of ice allows us to investigate iron oxides concentrations in atmospheric aerosol. Lanci et al. (2004) have shown that the small amount of atmospheric dust included in ice cores can acquire an IRM which can be precisely measured directly in ice samples and can be correlated with dust concentrations in the ice. If the correlation between ice magnetization and dust concentration is good, which implies uniform magnetic properties of dust, the slope of the regression line is taken as average magnetization on the dust fraction. The intercept of the regression line with the y-axis represents a background magnetization that in not correlated with dust concentration. Lanci and Kent, (2006) have found that this background magnetization is carried by nanometric-sized grains, known as superparamagnetic (SP), and it is mostly due to meteoric fallout. Magnetic measurements on ice also provide a measure of stable magnetization (e.g., Lanci and Kent, 2006) and the coercivity of remanence, which are discussed in this paper. Both of them provide information on the magnetic properties of the aerosol and may thus be a potential tracer
Detection of multiaxial magnetite by remanence effect on anisotropy of magnetic susceptibility
No full textA method is here proposed to detect the presence of magnetic particles with multi-axial anisotropy, based on the effect of remanence on the anisotropy of magnetic susceptibility. The theory for an ensemble of single-domain particles with uniformly distributed orientation is shown and applied to a lava sample from Stromboli and to mid-oceanic ridge basalts. Results are coherent with the magnetic properties of the test samples and have successfully identified a small fraction of magnetic susceptibility carried by multi-axial grains where this was expected. Theory and experiments have shown that the remanence may have a large effect on the anisotropy of magnetic susceptibility which, however, can be effectively removed by alternating field demagnetization. It is also suggested that alternating field demagnetization should be routinely applied in rock-fabric studies of highly magnetized rocks
Magnetostratigraphy of Eocene-Oligocene boundary sections in Umbria Italy: no evidence for short subchrons within chrons 12R and 13R
No full textThe anisotropy of magnetic susceptibility of uniaxial superparamagnetic particles: Consequences for its interpretation in magnetite and maghemite bearing rocks
No full textA simple model that provides a quantitative description of the magnetic susceptibility of superparamagnetic to stable single-domain uniaxial magnetic particles can be built in the framework of the theory of stochastic resonance. This model expands that of Mullins and Tile (1973) for superparamagnetic grains by considering the dependence of superparamagnetic susceptibility on the particle orientation and thus describes the anisotropy of magnetic susceptibility (AMS) of ensembles of superparamagnetic as well as single-domain particles. The theory predicts that on the contrary of stable single domain, the maximum anisotropy of superparamagnetic particles is parallel to their easy axis and shows that the AMS of ensembles of uniaxial particle is strongly dependent on the distribution of particle grain size, coercivity, measurement temperature, and frequency. It also explains why the inverse AMS pattern expected for stable single-domain particles is rarely observed in natural samples. We use examples of well-characterized obsidian specimens to show that, as predicted by the theory, in the presence of significant superparamagnetic contributions, the maximum susceptibility axis of AMS is directed along the preferential direction of particles easy axis
Magnetostratigraphic evidence that "tiny wiggles" in the oceanic magnetic anomaly record represent geomagnetic paleointensity variations
No full textForward modeling of thermally activated single-domain magnetic particles applied to First Order Reversal Curves
No full textTheoretical first-order reversal curves (FORCs) were generated by numerically solving a thermally activated Stoner-Wohlfarth model for assemblages of randomly oriented magnetic particles. The thermally activated Stoner-Wohlfarth model extends previous models based on Preisach theory. The new numerical simulations show that the shapes of reversal curves and the FORC distributions are significantly modified by the effect of the thermal energy only if superparamagnetic particles are predominant. However, most assemblages containing moderate amounts of superparamagnetic particles are hardly distinguishable from stable single-domain assemblages. The most relevant thermal effect is a reduction of coercivity that translates in a shift of the FORC distribution toward the origin. Not all of the distinctive characteristics previously predicted for superparamagnetic grain assemblages were confirmed by our calculations, and most of the observed modifications due to thermal effects can be considered minor. A direct comparison with hysteresis parameters shows that these simpler experiments can be equally effective in characterizing viscous and superparamagnetic particles
Meteoric smoke fallout revealed by superparamagnetism in Greenland ice
No full textMeteoric material reaching Earth contains an appreciable percentage of iron, much of which can be oxidized into nanometric-size particles produced by ablation and subsequent condensation in the atmosphere. New measurements of isothermal remanent magnetization (IRM) show that magnetic particles of extraterrestrial origin can be distinguished from terrigenous particles based on their smaller superparamagnetic (SP) size as inferred from magnetic relaxation and by the poor correlation of the SP fraction with dust contents. The magnetic relaxation data suggest that extraterrestrial magnetic particles are in the size range of about 7 – 17 nm, which is compatible with the expected size of condensed particles. The concentration of extraterrestrial material in Greenland ice was estimated from the magnetic relaxation data. Assuming an iron content typical of average chondritic composition, the results correspond to a particles concentration of 0.78 ± 0.22 ppb for Greenland, good agreement with results based on iridium concentrations in NGRIP ice samples
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